This invention relates to a toroidal continuously variable transmission (CVT) for vehicles.
Continuously variable transmissions (CVTs) for vehicles have recently been developed because of the smooth motion, easy operation, and improved fuel economy. Among the CVTs, there is well known a toroidal traction drive CVT (referred to hereinafter as a toroidal CVT) adapted for transmitting power using shear of an oil film. In general, the toroidal CVT includes an input disk on an input shaft, an output disk on an output shaft coaxial with the input shaft, and a plurality of power rollers disposed between the input and output disks in contact therewith.
The toroidal CVTs are classified depending on shape thereof into a full toroidal and a half toroidal. In the full toroidal CVT, there occurs a thrust or axial force applied to the power rollers. On the other hand, the half toroidal CVT suffers from application of a thrust to the power rollers, necessitating bearing for carrying the thrust. The characteristic of the bearing significantly affects operating efficiency of the half toroidal CVT. However, in the half toroidal CVT, intersection of two tangent lines extending from two contact points between the input and output disks and the power roller forms a locus that is located near a rotation axis of the power roller. This causes a reduced spin loss in the half toroidal CVT as compared with a spin loss in the full toroidal CVT. The half toroidal CVT has been selectively used by taking account of the balance between the features of the full toroidal CVT and the half toroidal CVT. The speed change operation by the half toroidal CVT is conducted by slightly displacing a power roller support member (referred to hereinafter as a trunnion) in a direction perpendicular to the rotation axis of the power roller and the common rotation axis of the input and output disks. The displacement causes a side-slip force to thereby generate a slant rolling force.
Japanese Patent Application First Publication No. 9-004688 discloses a toroidal CVT including an input shaft receiving a torque from an engine, an input disk connected with the input shaft, an output disk connected with an output gear disposed coaxially with the input shaft, and input and output bearings supporting the input shaft and the output gear within a transmission casing, respectively. A plurality of power rollers are disposed within a toroidal cavity formed between opposed surfaces of the input and output disks. A loading cam is disposed between the input shaft and the input disk and applies an axial force to the input and output disks corresponding to the torque. The torque is transmitted from the input shaft to the output gear via the input disk, the power rollers and the output disk. A taper-roller bearing is used for each of the input and output bearings.
In the toroidal CVT of the earlier technique described above, the taper-roller bearing used for each of the input and output bearings has an outer race contact angle of less than 45 degrees. The taper-roller bearing carries a radial load rather than a thrust load due to the outer race contact angle. It will be required to increase the size of the bearing in order to carry both of the thrust load and the radial load and obtain satisfactory rolling-fatigue life of the bearing. The dimensional increase of the bearing will cause enlargement of the entire CVT.
The input and output bearings carry both of a thrust load generated by application of the axial force by the loading cam and a radial load generated by meshing engagement of the output gear with the corresponding engaging member. The axial force of the loading cam is remarkably large, namely, not less than ten times the radial load, in order to frictionally transmit the torque between the input and output disks and the power rollers.
Accordingly, it seems appropriate to use in the toroidal CVT a thrust taper-roller bearing having an outer race contact angle of not less than 45 degrees. Referring to FIG. 7, load acting on the mutual contact portion where a raceway of an outer race and a tapered roller of a taper-roller bearing are in contact with each other, is explained. In FIG. 7, R1, R2, R3 and R4 denote an inner race, an outer race, a tapered roller and a raceway of outer race R2 of the taper-roller bearing, respectively. Load Fc acing on the mutual contact portion of raceway R4 of outer race R2 and roller R3 is expressed by the following equation:
xe2x80x83Fc=Fc1+Fc2=Fa/sinxcex1+Fr/cosxcex1xe2x80x83xe2x80x83(1)
where Fa is a thrust load, Fr is a radial load, and xcex1 is an outer race contact angle. Outer race contact angle xcex1 is formed by the rotation axis of the taper-roller bearing and the mutual contact between roller R3 and raceway R4 of outer race R2. Here, a relationship between thrust load Fa and radial load Fr is expressed as Fa greater than  greater than Fr. If outer race contact angle xcex1 becomes larger, load Fc will decrease. In this case, the rolling-fatigue life of the taper-roller bearing can be improved.
However, if outer race contact angle xcex1 becomes larger, an increase ratio of load Fc to radial load Fr will become higher. This will cause rigidity of the taper-roller bearing in the radial direction to be lowered. Therefore, even if a small load generated by the meshing engagement of the output gear and the corresponding gear member acts on the taper-roller bearing, the inner race will be eccentrically largely displaced relative to the outer race so that the input and output disks will be placed in offset positions relative to the power rollers. This will adversely affect controllability of speed change of the CVT.
In the consideration of the characteristic of the thrust taper-roller bearing as explained above, the toroidal CVT of the earlier technique employs a radial taper-roller bearing having outer race contact angle xcex1 of less than 45 degrees for each of the input and output bearings. In this case, however, relatively large thrust load Fa will be amplified to produce larger load Fc. Therefore, the radial taper-roller bearing must be enlarged in size in order to provide the satisfactory rolling-fatigue life.
An object of the present invention is to provide a toroidal continuously variable transmission (CVT) which is capable of exhibiting satisfactory rolling-fatigue life of input and output bearings and maintaining high speed-change controllability by using the input and output bearings having reduced size.
According to one aspect of the present invention, there is provided a toroidal continuously variable transmission for a vehicle engine, comprising:
a casing;
an input shaft rotatably disposed within the casing, the input shaft being adapted to receive a torque from the engine;
an input disk connected with the input shaft;
an output disk cooperating with the input disk to form a toroidal cavity between opposed surfaces thereof;
a power roller rotatably disposed in the toroidal cavity;
a loading member applying a force corresponding to the torque to the input and output disks so as to make frictional contact between the power roller and the opposed surfaces of the input and output disks;
an output shaft receiving the torque via the power roller and the output disk;
an input bearing rotatably supporting the input shaft within the casing; and
an output bearing rotatably supporting the output shaft within the casing,
each of the input and output bearings comprising a taper-roller bearing having a rotation axis and a radial bearing arranged parallel to the taper-roller bearing with respect to the rotation axis, the taper-roller bearing comprising an inner race, an outer race and a plurality of tapered rollers contacted with the inner and outer races, the taper-roller bearing having an outer race contact angle of not less than 45 degrees between the rotation axis and the mutual contact of the outer race and the tapered rollers.
According to a further aspect of the present invention, there is provided a toroidal continuously variable transmission, comprising:
a casing;
an input shaft rotatably disposed within the casing;
an input disk coaxially connected with the input shaft;
an output disk arranged in coaxial and opposed relation to the input disk, the output disk cooperating with the input disk to form a toroidal cavity between opposed surfaces thereof;
an output shaft coaxially and rotatably connected with the output disk;
a power roller rotatably disposed within the toroidal cavity in contact with the opposed surfaces of the input and output disks;
a loading member applying a thrust force to the input and output disks;
an input bearing rotatably supporting the input shaft within the casing; and
an output bearing rotatably supporting the output shaft within the casing,
each of the input and output bearings comprising a thrust bearing having a rotation axis and a radial bearing arranged parallel to the thrust bearing with respect to the rotation axis, the thrust bearing comprising an inner race, an outer race on which the thrust force applied by the loading member acts, and a plurality of tapered rollers in contact with the inner and outer races, the thrust bearing having an outer race contact angle of not less than 45 degrees between the rotation axis and the mutual contact of the outer race and the tapered rollers.